Bowing coefficient representation of curvature of geographic features

ABSTRACT

The degree to which a linearly extending feature, such as a road, curves is indicated using a bowing coefficient. The bowing coefficient at a given location along a linearly extending feature is determined by comparing the distance along the feature between two points on either side of the given location (or an approximation of the distance) to a straight-line distance between these same two points. Bowing coefficient data can be used by various vehicle systems that require information about the curvature of linearly extending features, such as roads upon which the vehicle is traveling.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to geographic data and moreparticularly, the present invention relates to a way to represent howmuch a linearly extending geographic feature (such as a road) curves byusing a bowing coefficient.

[0002] Various new vehicle safety systems have been developed thatprovide warnings to the vehicle driver or that modify operation of thevehicle (or component thereof) based upon conditions around the vehicleor other factors. Examples of some of these new vehicle safety systemsinclude automatic headlight aiming, automatic (or adaptive) cruisecontrol, obstacle warning, curve warning, intersection warning, lanedeparture warning, collision warning, and adaptive transmission shiftcontrol. Some of these vehicle safety systems use sensor equipment(e.g., radar and cameras) to detect the current state of the roadway andenvironment around the vehicle. Some of these vehicle safety systems usedigital map data as a component. Digital map data are used to identifythe location and shape of the road ahead of and around the vehicle.Digital map data can also be used to identify relatively permanentobjects or features along the roads.

[0003] Included among the types of digital map data used by some ofthese vehicle systems are data that indicate the curvature of the road.In some digital map databases, the curvature of a road at given locationis indicated by a radius of curvature value (or inverse thereof). Somevehicle safety systems use these radius of curvature data to modifyoperation of the vehicle. For example, an automatic cruise controlsystem in a vehicle uses the data that indicate curvature of a road atthe location along the road at which a vehicle is traveling to determinean acceptable speed range for the vehicle. After determining anacceptable speed range, the automatic cruise control application adjuststhe speed of the vehicle if necessary.

[0004] The map database used in this type of vehicle safety systemincludes data indicating the positions of points along roads includingdata indicating the radius of curvature at the various points along theroads. When forming this type of map database, curvature values arecomputed using data that identify the coordinates of a series of points(referred to a “shape points”) through which the road passes.

[0005] Although using computed values of radius of curvature torepresent road shape is satisfactory for some vehicle applications,there is room for improvement. The computed radius of curvature valuescan be affected by the type of function (e.g., piecewise linear,b-spline, etc.) that is used to approximate the given set of shapepoints. Furthermore, small changes in the approximating function mayproduce large changes in the curvature value. Also, small changes in thedata point locations themselves may produce a large change in a computedradius of curvature.

[0006] Accordingly, there is a need for another way to represent roadgeometry in a geographic database.

SUMMARY OF THE INVENTION

[0007] To address these and other objectives, the present inventioncomprises a way to represent the curvature of a linearly extendingfeature, such as a road. The curvature of a linearly extending feature,such as a road, is represented using a bowing coefficient. The bowingcoefficient at a given location along a linearly extending feature isdetermined by comparing the distance along the feature between twopoints on either side of the given location (or an approximation of thedistance) to a straight-line distance between these same two points.Bowing coefficient data can be used by various vehicle systems thatrequire information about the curvature of linearly extending features,such as roads upon which the vehicle is traveling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a diagram used to illustrate some of the terminology inthis specification.

[0009]FIG. 2 is a diagram used to illustrate the bowing coefficient atdifferent locations along a road.

[0010]FIG. 3 is a diagram that illustrates how the bowing coefficientvaries with curvature.

[0011]FIG. 5 is an enlarged portion of the area encompassed within thedotted circle in FIG. 4.

[0012]FIG. 6 is a diagram illustrating operation of an alternativeembodiment.

[0013]FIG. 7 is a diagram illustrating operation of another alternativeembodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0014] I. Definitions

[0015] The following terms are illustrated in FIG. 1.

[0016] “Chord” refers to the straight-line segment joining any twopoints on a road segment.

[0017] “Chord length” refers to the length of the straight-line segmentjoining any two points on a road segment.

[0018] “Arc” refers to the portion of a road segment between any twopoints on the road segment.

[0019] “Arc length” refers to the length (or approximation thereof) ofthe portion of a road segment between any two points on the roadsegment.

[0020] As used in this specification, the word “curvature” refers to thegeneral property of a linearly extending feature being curved and is notrestricted to a mathematical definition, except as otherwise indicated.The word “curvature” in the phrase “radius of curvature” is understoodto have its mathematical meaning.

[0021] II. Overview

[0022]FIG. 2 shows a road segment 10. Several chords 12 have been drawn(in dashed lines) between pairs of points 14 on this segment 10. FromFIG. 2, it can be seen that the portions of the road segment, which arenearly straight, have arc lengths which are nearly equal to thecorresponding chord lengths. Conversely, portions of the road segmentthat are curved have much larger arc lengths than corresponding chordlengths. This means that for relatively straight portions of a roadsegment, the ratio of arc length to chord length is close to 1, and forrelatively curved portions of a road segment, the ratio of arc length tochord length is significantly greater than 1. In other words, the ratioof arc length to chord length is an indication of the local curvature ofthe road segment.

[0023] This relationship between curvature of a road segment, the chordlength, and the arc length is described by the following.$\left. \frac{A}{C} \right.\sim\kappa$

[0024] where C is the chord length, A is the arc length and κ is thecurvature (i.e., using the mathematical definition of “curvature”).

[0025] For purposes of this specification, the ratio of A to C isreferred to as the “bowing coefficient.” The “bowing coefficient.” is ameasure of how much a portion of road segment between two points bendsor bows out from the straight line joining these same two points.

[0026]FIG. 3 illustrates the variation in the bowing coefficient withcurvature. FIG. 3 shows three alternative paths 30, 32, and 34 along aroad segment 38. Each of these different paths represents a differentalternative road configuration between the points 40 and 42. As shown byFIG. 3, the greater the curvature, the greater the bowing coefficient.

[0027] III. Implementation

[0028] The bowing coefficient can be used by various vehicle systems andapplications that use data that represent road geometry. For example,the bowing coefficient can be used by an automatic cruise controlapplication. Use of the bowing coefficient by an automatic cruisecontrol application is described in connection with FIGS. 4 and 5.

[0029] In FIG. 4, a vehicle 50 having an automatic cruise control systemtravels along a road segment 52. The automatic cruise control systemobtains data indicating the position of the vehicle with respect to theroad as represented by data contained in a map database. This functioncan be performed using known vehicle positioning technology, such asGPS, dead-reckoning, and so on.

[0030] As the vehicle 50 travels along the road segment 52, theautomatic cruise control application in the vehicle 50 adjusts the speedof the vehicle based on the bowing coefficient of the road. In FIG. 4,the arrows 54 indicate the instantaneous positions of the vehicle. Thechords 58 corresponding to these positions are also shown.

[0031] According to one embodiment, at each position 54, the automaticcruise control application in the vehicle selects two points straddlingthe position at which the vehicle is located. The automatic cruisecontrol application then computes the chord length C, the arc length Aand the bowing coefficient using the two points. As indicated in FIG. 5,as the vehicle moves into the curve, the bowing coefficient increases,and as the vehicle comes out of the curve the bowing coefficientdecreases, as is expected. Using these computed values of the bowingcoefficients, the automatic cruise control application in the vehicleadjusts the vehicle speed accordingly.

[0032] The selection of the two points that straddle the position of thevehicle is configurable so that the bowing coefficient derived therefromis suitable for the application by which it is used. As an example, thedistance (or distance range) of each of the two points from the positionof the vehicle is configurable.

[0033] As illustrated in FIG. 5, some of the arc lengths and chordlengths overlap for successive positions at which the bowing coefficientis determined. As previously indicated, the selection of the points usedto determine the bowing coefficient is configurable so that anappropriate measure of the curvature is obtained. There is no constraintthat the arc lengths and chord lengths not overlap.

[0034] IV. Alternative Embodiments

[0035] A. Using Previously Calculated Bowing Coefficient Data

[0036] According to one alternative embodiment, a vehicle safety system,such as automatic cruise control, uses a map database that includesbowing coefficients for points along roads. According to thisalternative embodiment, the values for the bowing coefficient at pointsalong roads are computed in advance by the database developer and storedin the geographic database. The geographic database, which includes thecomputed values for the bowing coefficient for points along roads, isinstalled in the vehicle and used by vehicle safety systems, such asautomatic cruise control.

[0037] An advantage of storing computed values of the bowing coefficientin the map database used by the vehicle safety system is that iteliminates the need to compute these values in the vehicle.

[0038] B. Calculating Bowing Coefficients

[0039] There are several different ways that the bowing coefficient canbe calculated. One way to calculate a bowing coefficient is described inconnection with FIG. 6.

[0040]FIG. 6 shows a portion of a road 60. Along the road are shapepoints 62, 64, 66, 68, and 70. These shape points are points at whichthe geographic coordinates of the location of the road are known. Thegeographic coordinates at these locations may be determined usingvarious data collection procedures. For example, the geographiccoordinates at these positions may be determined using GPS equipment.Alternatively, the geographic coordinates at these positions may bedetermined from aerial photographs.

[0041] According to one alternative embodiment, a value of the bowingcoefficient is determined for each shape point. According to thisalternative embodiment, a value of the bowing coefficient at a givenshape point can be approximated by comparing the sum of the distancesfrom the given shape point to the two shape points immediately adjacentto the given shape point to the straight-line distance between the twoadjacent shape points. According to this alternative, the arc length isapproximated by using two “chord lengths.”

[0042] For example, to determine the bowing coefficient for the shapepoint labeled 66, the distance from the shape point 64 to the shapepoint 66 is summed with distance from the shape point 66 to the shapepoint 68. Then, this sum is divided by the distance from the shape point64 to the shape point labeled 68 in order to determine the bowingcoefficient at the shape point 66. Bowing coefficients for the rest ofthe shape points can be determined in a similar manner. The values ofthe bowing coefficients can then be stored with the coordinates of theshape points in a geographic database. Alternatively, using this method,bowing coefficients can be computed on-the-fly, as needed, by anapplication in the vehicle during runtime.

[0043] According to another alternative, an approximation of the bowingcoefficient at a given shape point can be determined by taking intoaccount the distances to additional shape points beyond thoseimmediately adjacent to the given shape point. FIG. 7 illustrates thisembodiment. FIG. 7 shows the same portion of road that is shown in FIG.6. In FIG. 7 the bowing coefficient at the point 66 is approximated byfirst summing the straight-line distances from the point 62 to the point64, the point 64 to the point 66, the point 66 to the point 68, and thepoint 68 to the point 70. The sum of these distances is then divided bythe straight-line distance between the point 62 and the point 70 therebyyielding the bowing coefficient at the point 66. According to thisalternative, the arc length is approximated by using four “chordlengths.”

[0044] In FIG. 7, two points were selected on either side of the pointat which the bowing coefficient was determined. Alternatively, anynumber of points can be selected on either side of the point at whichthe bowing coefficient is determined.

[0045] According to still another alternative, the distances from thepoint along a road at which the bowing coefficient is determined to thepoints adjacent thereto used in determining the bowing coefficient canbe actual distances as-the-vehicle-travels. The actual distancesas-the-vehicle-travels can be collected using odometer or speed pulsedata or determined from examination of aerial or satellite photographs.

[0046] C. Other Alternatives

[0047] In another alternative, the values of the bowing coefficient canbe computed on the fly in the vehicle using data representing the roadgeometry, such as shape point data.

[0048] The new method described here can be used in combination with theexisting radius of curvature approach to improve the robustness andeffectiveness of various vehicle applications.

[0049] In the embodiments described above, bowing coefficient data wereused to represent the curvature of roads. In alternative embodiments,bowing coefficient data can be used to represent the curvature of otherkinds of linearly extending features, such as rivers, railroad tracks,boundaries, trajectories, ferries, and so on.

[0050] V. Advantages

[0051] Several advantages follow from using bowing coefficients torepresent curvature. First, using bowing coefficients to represent thecurvature of linearly extending geographic features, such as roads, doesnot involve the computation of radius of curvature values which areprone to large errors. Further, using bowing coefficients to representthe curvature of linearly extending features is less computationallyintensive than using radius of curvature values. In addition, bowingcoefficients can be derived from data that are stored as a set of shapepoints (piecewise linear approximation), polynomial spline controlpoints, etc.

[0052] It is intended that the foregoing detailed description beregarded as illustrative rather than limiting and that it is understoodthat the following claims including all equivalents are intended todefine the scope of the invention.

I claim:
 1. A method of representing how much a road curves comprising:selecting two points along the road; and comparing an approximation of adistance along the road between the two points to a straight-linedistance between the two points; whereby a result of said comparing isan indication of how much the road curves between the two points.
 2. Themethod of claim 1 further comprising: using said indication of how muchthe road curves to adjust a speed of a vehicle.
 3. The method of claim 1further comprising: storing said indication of how much the road curvesin a geographic database.
 4. The method of claim 1 wherein said step ofcomparing is performed using shape point data that represent geographiccoordinates at locations along the roads.
 5. The method of claim 1wherein said step of comparing is performed by an application in avehicle.
 6. A method of representing road geometry comprising: atselected locations along a road, determining a bowing coefficient,wherein the bowing coefficient at each of said selected locationscorresponds to a comparison between an approximation of a distance alongthe road between two points on the road on either side of the selectedlocation and a straight-line distance between the two points; and usingthe bowing coefficient as an indication of curvature of said roadbetween said two points.
 7. The method of claim 6 further comprising:storing data indicating said bowing coefficient in a geographic databasethat represents said road.
 8. A method of operating a vehicle alongroads comprising: with a software application in said vehicle, accessinga database containing data that represent said roads; determining alocation of said vehicle with respect to the roads as represented bysaid database; and using data that indicate a bowing coefficient at eachof a plurality of locations along said roads to adjust operation of saidvehicle.
 9. The method of claim 8 wherein a speed at which said vehicleis moving is reduced as said vehicle approaches a portion of said roadsat which said bowing coefficient is relatively higher.
 10. The method ofclaim 8 wherein a speed at which said vehicle is moving is increased assaid vehicle approaches a portion of said roads at which said bowingcoefficient is relatively lower.
 11. A method of forming a geographicdatabase comprising: comparing an approximation of a distance along aroad segment between two points to a straight-line distance between saidtwo points; and storing in said geographic database a result of saidcomparing as an indication of curvature of said road segment betweensaid two points.
 12. A geographic database formed according to theprocess of claim
 11. 13. A method of operating a vehicle along roadscomprising: relating a position of said vehicle on a road to a datarepresentation of the road contained in a geographic database, whereinthe data representation of the road includes an indication of curvatureat locations along the road, and wherein the indication of curvatureincludes a comparison between an arc length between points along theroad and a chord length between the points; and adjusting operation ofthe vehicle based on the indication of curvature corresponding to saidposition of said vehicle.
 14. A method of operating a vehicle alongroads comprising: with a software application in said vehicle, computinga relationship between a distance between two points along a road and astraight-line distance between said two points; determining a locationof said vehicle with respect to said road; and adjusting operation ofsaid vehicle using said computed relationship.
 15. The method of claim14 wherein said computing step is performed using data contained in ageographic database located in said vehicle.
 16. A method of operationfor an application in a vehicle comprising: accessing a geographicdatabase that contains data that represent roads upon which the vehicleis traveling; determining a position of said vehicle with respect tosaid data that represent roads upon which the vehicle is traveling; andusing data that represent bowing coefficients at locations along saidroad to adjust operation of said vehicle.